There are three major theories of First Language Acquisition, however, I will be focusing on the Behaviousim Theory(1950) by B.F. Skinner and the Innatism Theory by Noam Chomsky (1959). 

Behaviourism Theory: Say What I Say

Skinner believes that children imitate the language produced by others around them or, their attempts to reproduce they have heard. He believes that positive reinforcement or praise impacts how effective the acquisition of this first language will be. He also argues that these children are encouraged by their environment as they continue to imitate and practice sounds and patterns until they are able to form correct language use. It can be said that the quantity and quality of a language that a child hears along with consistent reinforcement by others in the environment will lead to a normal child being fully able to acquire a language. 

Innatism Theory: It's All in Your Mind 

On the other hand, Noam Chomsky(1959), believes that all humans are fundamentally innate and the same universal principle underlines all of them. He went on to say that children are biologically programmed for language and language develops the same way other biological forms are developed. For example, once a baby is properly taken care of and nourished he will learn to walk as long as he is given the freedom of movement in his environment, he does not have to be taught. It can be said that the environment is only a basic platform as it relates to the acquiring of a language the biological aspect of a child determines how they acquire language and what they do with it when it is acquired. All children are born with a specific innate ability to discover for themselves the underlying rules of a language.